. I have truncated it at a random point, so I doubt is decodable like that, but you get the point... What is that encoding?

Also, wouldn't hurt to know where in the docs they explicitly say its base64 (unless it's the standard encoding for MIME?).

UPDATE: So in the end there was some bad luck involved. I have 5 mails like this, and turns out that the first one is malformed, for some unknown reason. After moving on to the other ones, I am able to decode all of them with the suggested approaches in the answers. Thank you all!